An important aim in elevator systems is to maximize passenger safety. Free fall of the elevator car must be prevented and the motion must not reach an uncontrolled acceleration and consequent uncontrolled deceleration of motion. Even if a sudden stop occurs with a relatively low kinetic energy, passengers may suffer injuries.
When the elevator car has stopped at a landing or also if it has stopped between floors for some reason, the machine brake is triggered, in other words, the machine brake prevents or at least it should prevent both upward and downward movement of the car from the stopping position. The machine brake is generally arranged to engage the traction sheave.
When the machine brake is on and the car is standing still, generally also with the car doors open, the car should absolutely remain immovable. Only small movements due to changes in car load consequent rope elongations are allowed. These movements are of the order of 1-2 cm.
However, the machine brake may suffer faults that prevent its perfect operation. Maintenance may have failed, adjustments may be wrong, the brake shoes or only one of the brake shoes may be defective. It is also possible that electric release of the machine brake does not work and so the brake is continuously engaged. Therefore it may happen that, while the motor is powerful enough to move the elevator car, the brake is continuously chafing the traction sheave and after a sufficiently long time of operation the brake becomes inoperative and is no longer able to keep the car immovable when it should.
In new elevators, the above-mentioned problems have been eliminated by using equipment that in principle could also be used in old elevators already in use. However, this would require such large changes and additions to the structures of old elevators that it would be necessary to carry out a complete re-inspection of the elevators. In practice, the entire elevator would have to be renewed to an extent such that building a completely new elevator might even be more remunerative in long-term economics.